A lithographic apparatus is a machine that applies a desired pattern onto a substrate, typically onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of one or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In a lithographic apparatus, the size of features that can be imaged onto the substrate may be limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to be able to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, e.g. of around 13 nm. Such radiation is termed extreme ultraviolet (EUV) or soft x-ray, and possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings.
The source of EUV radiation is typically a plasma source, for example a laser-produced plasma or a discharge source. A common feature of any plasma source is the production of fast ions and atoms, which are expelled from the plasma in all directions. These particles can be damaging to the collector and condenser mirrors which are generally multilayer mirrors or grazing incidence mirrors, having fragile surfaces. This surface is gradually degraded due to the impact, or sputtering, of the particles expelled from the plasma and the lifetime of the mirrors is thus decreased. The sputtering effect is particularly problematic for the radiation collector or collector mirror. The purpose of this mirror is to collect radiation which is emitted in all directions by the plasma source and direct it towards other mirrors in the illumination system. The radiation collector is positioned very close to, and in line-of-sight with, the plasma source and therefore receives a large flux of fast particles from the plasma. Other mirrors in the system are generally damaged to a lesser degree by sputtering of particles expelled from the plasma since they may be shielded to some extent.
In the near future, extreme ultraviolet (EUV) sources may use tin (Sn) or another metal vapor to produce EUV radiation. This tin may leak into the lithographic apparatus, and be deposited on mirrors in the lithographic apparatus, e.g. the mirrors of the radiation collector. The mirrors of such a radiation collector may have a EUV reflecting top layer of, for example, ruthenium (Ru). Deposition of more than approximately 10 nm tin (Sn) on the reflecting Ru layer will reflect EUV radiation in the same way as bulk Sn. The overall transmission of the collector may decrease significantly, since the reflection coefficient of tin is much lower than the reflection coefficient of ruthenium. In order to prevent debris from the source or secondary particles generated by this debris from depositing on the radiation collector, contaminant barriers may be used. Though such contaminant barriers or traps may remove part of the debris, still some debris will deposit on the radiation collector or other optical elements.
In order to remove unwanted deposition, cleaning methods have been discussed, including for instance hydrogen radical cleaning, such as for instance described in International Patent Application Publication No. WO2008002134. In this way, H2 and hydrogen radicals are introduced in at least part of the lithographic apparatus. WO2008002134 describes a cleaning arrangement for use in an EUV lithographic apparatus, for example an EUV lithographic apparatus with a Sn source. The cleaning arrangement includes a gas source for a hydrogen containing gas and a hydrogen radical source. The hydrogen radical source is a source of (UV) radiation which induces photo dissociation of the hydrogen (this could also be the EUV source) or a hot filament that generates hydrogen radicals. Radicals may reduce Sn oxides (if present) and form volatile hydrides of Sn deposition and/or carbon deposition. In this way, the cleaning arrangement can be used to clean optical elements from Sn and/or C deposition.